This invention relates to improvements in suspension systems for supporting a plurality of high voltage electrical conductors forming part of a transmission line in a predetermined spaced pattern relative to one another.
The traditional form of high or extra high voltage transmission line support structure included a rigid framework of latticed steel having rigid crossarms which supported the phases of conductors at the ends of the necessary insulator assemblies.
Approximately twenty years ago, a sometimes more useful or efficient structure consisting of a rigid crossarm supported on two guyed masts in V or Portal form, became popular. For lower voltage applications, these towers are small enough to be totally assembled on a flat area near the site and raised or rotated into position with a small crane or gin pole assembly. However, for higher voltage applications, the crossarms must be made relatively large with the result that they are heavy and unwieldy and pose very serious construction problems.
An alternative solution has been to replace the conductor-supporting crossarm structure or bridge with a steel cable system suspended between a pair of masts. There has been a continuing and present trend to the use of these "crossarmless" towers, the first to be used in a major operating line being the cross rope suspension structure (the Chainette) on Hydro-Quebec's Third James Bay to Montreal 735 KV Line. Potential advantages of the cross armless towers include lower construction and material supply costs as well as other effects which result from the ability to bring the conductor phases closer together. In this regard it might be noted that there are no grounded steel components between the phases from which one must maintain very large clearances. This reduced phase spacing reduces the necessary width of the right-of-way and also lowers the electrical reactance, thus permitting greater power transfer capacity under stable conditions.
There has been a substantial degree of acceptance by those skilled in this art of the principle of suspending the conductor phases from a flexible support assembly which in itself is supported between two relatively fixed points. These points will normally, but not necessarily, be the upper ends of two guyed masts but could also be the tops of two arms of a rigid tower structure.
The next development in this area was to rearrange the conductors from a flat configuration (as seen looking along the conductors) into an inverted Delta or triangular array. The advantages of this configuration include a still further reduced right-of-way requirement, coupled with the electrical advantages of even less reactance and a significant reduction in electric field effects at the ground or pedestrian level as one moves away from the center of the line. The primary flexible suspension assemblies which have been devised to date are the suspended inverted Delta or Cluster (Italian notation) or the Suspended T, which has been tested to a limited extent in Germany. These suspension assemblies must be strong enough to resist the loads developed in the conductor systems, which may be any of or combinations of:
(a) Vertical loads of the weight of the conductor plus any designated ice or snow cover or special vertical loads produced by construction or maintenance practices. PA0 (b) Transverse loads produced by wind blowing on the conductors, either bare or ice covered. PA0 (c) Longitudinal loadings that result from either unbalanced ice or wind loads on adjacent spans, or failure containment conditions whereby the tower must resist the longitudinal forces produced by conductor breakage or failure of an adjacent tower.
The above-noted loads and particularly the vertical loads resulting from ice on the conductor produce a very large strength demand on the end insulators of the insulator array, i.e. those insulators which, in the Delta arrangement for example, extend from opposing corners of the Delta upwardly and outwardly to the relatively fixed points noted above which are usually at the tops of the two guyed masts. High strength insulator assemblies are very expensive and they can cost more in certain instances than the supporting mast systems. Furthermore, the large loadings in the end insulators, particularly those produced by vertical loadings, are applied to the suspension points and can be the controlling loads for the mast/guy system. Further disadvantages of the above system are that the integrity of the entire structure depends on the integrity of the end insulators referred to above, i.e. failure of one of them would involve collapse of the entire structure, and that the structure becomes unstable in the event of a broken guy wire.
Of the several problems noted above, the primary problem to be faced is the fact that in these structural arrangements, the insulators and associated hardware of the suspension assembly can represent a large part of the material cost of the total structure, including masts and guy wires. High strength insulators are very expensive in the numbers required for these suspension systems.